Reliable and efficient compressors, such as centrifugal compressors, are often utilized in a myriad of applications and industrial processes (e.g., petroleum refineries, offshore oil production platforms, and subsea process control systems) to compress a process fluid. Centrifugal compressors may include one or more compressor stages, where each compressor stage may generally include an impeller and a diffuser. Developments in compressor designs have resulted in improved centrifugal compressors with compressor stages capable of achieving high compression ratios. Accordingly, the centrifugal compressors incorporating the high-pressure ratio compressor stages may compress the process fluid to increased pressures. The increased pressures of the process fluid, however, may apply reaction and/or excitation forces to a rotary shaft of the centrifugal compressor that may often generate or increase vibrations of the rotary shaft and lead to rotordynamic instability. The increased vibrations and the resulting rotordynamic instability may cause the rotary shaft to contact or impact one or more surrounding components (e.g., stationary components), thereby damaging the rotary shaft and/or the surrounding components.
In view of the foregoing, conventional centrifugal compressors may often incorporate a damper seal in a flowpath of the centrifugal compressor to reduce or dampen the vibrations of the rotary shaft. The damper seal may often include an annular member defining a plurality of openings along an inner radial surface thereof, and may be disposed about the rotary shaft such that the inner radial surface and the rotary shaft define a radial clearance or gap therebetween. In operation, the process fluid may flow axially through the clearance from an inlet to an outlet of the damper seal and become entrained or trapped in the openings disposed along the inner radial surface thereof. The entrainment of the process fluid in the openings may generate damping forces that may overcome cross-coupling stiffness and/or destabilizing forces acting upon the rotary shaft to thereby improve the rotordynamic stability.
For the damper seal to be effective, a minimum pressure differential between the inlet and the outlet of the damper seal may often be required to drive the process fluid through the clearance. Further, for improved effectiveness in damping the vibrations, the damper seal may often be disposed at or proximal a middle or mid-span of the rotary shaft, where the maximum deflection and/or vibrations of the rotary shaft may occur. For example, in some compressor configurations, such as a back-to-back centrifugal compressor, the damper seal may be disposed proximal the mid-span of the rotary shaft between a first compression assembly and a second compression assembly, and the pressure differential may be provided between a discharge of the first compression assembly (e.g., low-pressure area) and a discharge of the second compression assembly (e.g., high-pressure area). In other compressor configurations, such as a double or dual-flow centrifugal compressor, however, a pressure differential may not exist at the mid-span of the rotary shaft between the first and second compression assemblies. For example, the first and second compression assemblies of the dual-flow centrifugal compressor may be operated in parallel and have substantially equal discharge pressures. Accordingly, while the damper seals have proven to be effective in improving rotordynamic stability, the damper seals may have limited utility and/or applicability in the dual-flow centrifugal compressor due to the absence of a pressure differential across the damper seal located between the compression assemblies thereof.
What is needed, then, is an improved damper seal capable of reducing vibrations and improving rotordynamic stability of a rotary shaft in a dual-flow centrifugal compressor.